The implementation for aircraft, in particular for drones, of a non-cooperative radar function for detecting aerial obstacles is essential to allow the insertion of autopiloted aircraft into the unsegregated airspace. It participates in the obstacle detection and avoidance function known by the name “Sense and Avoid”.
The field of application of the invention is notably that of short- and medium-range radars, not requiring a large antenna area, but requiring very good angular precision. This is the case in particular for radars intended for the “Sense & Avoid” function.
Certain problems encountered for radar implementation of “Sense & Avoid” type may be encountered in other contexts in so far as the constraints weighing on the definition of the radar are analogous.
Radar architectures with simultaneous and continuous emission and reception are considered for the applications concerned, allowing best use to be made of the mean power available at the level of the solid-state power amplifiers, notably according to the solutions set forth in the French patent applications with the filing numbers FR 09 03799 and FR 09 04394.
One problem to be solved is the defining of a continuous waveform and of an associated detection processing allowing at one and the same time:                non-ambiguous processing in the Doppler domain;        a significant instrumented distance domain;        maximum efficiency of the waveform;        sufficient distance resolution to make it possible to limit the dynamics of the signal received on the ground clutter, to filter the near-distance rain echoes and to reduce the impact of emission and reception leakages on the sensitivity of the radar.        
The solutions generally implemented in respect of such a problem consist in using waveforms that are modulated linearly in the form of a frequency ramp, of FMCW type. It is also possible to provide time intervals between the ramps.
In a conventional manner, the processing of the distance measurements, on reception, consists, for each ramp, in demodulating the signal received by the image of the signal emitted, in sampling the signal thus demodulated, and then in performing a digital Fourier transform on the sampled signal. This first step makes it possible to separate the targets in the distance domain. A second Fourier transform performed from ramp to ramp makes it possible to separate the targets by Doppler processing in the speed domain. To be able to use such a waveform, the instrumented range of the radar must be such that at the maximum detectable distance the propagation delay for the outward-return journey for the emitted wave is much less than the duration of a ramp. The reception ramp then practically coincides with the emission ramp. In this case, the efficiency of the waveform is around 1 and almost all the available emission power may be used to ensure the radar range budget.
On the other hand, when the propagation delay for the outward-return journey is more significant than the duration of a recurrence, waveform efficiency is no longer ensured. This is typically the case for radars implementing the “Sense & Avoid” function. In particular, in this case two conflicting constraints appear. It is indeed impossible to simultaneously optimize the efficiency of the radar waveform and to minimize the noise related to emission leakage.